1. Field of the Invention
Embodiments of the invention relate generally to photobioreactors and associated photobioreaction methods and applications. More particularly, embodiments of the invention are directed to optofluidic photobioreactor apparatuses and associated methods and applications, in which light is delivered to a photosynthetically-active entity (e.g., bacteria, algae, or other photosynthetic microorganism) through the evanescent radiation field from the surface of a waveguide adjacent the microorganism. Non-limiting embodied applications of the invention pertain to the delivery of said evanescent radiation to said microorganisms to directly or indirectly produce fuels, chemicals, and/or biomass such as, but not limited to, algae, that can be further processed to produce chemicals such as, but not limited to, fuel.
2. Technical Background
The conversion of solar energy to fuel through the cultivation of photosynthetic algae and cyanobacteria relies critically on light delivery to microorganisms. Conventional direct irradiation of a bulk suspension leads to nonuniform light distribution within a strongly absorbing culture, and related inefficiencies.
Growing concern over global climate change and the rising cost of fossil fuels has led to substantial investment and research into alternative fuel sources. For this reason, bioenergy approaches have been developed to produce fuels such as ethanol, methanol, hydrogen and diesel. In order to compete with fossil fuels, however, producing biofuels require large feedstock volumes of inexpensive biomass. Although many feedstocks have been explored, including used cooking oil, food crops, and biowastes, most suffer from low net energy benefit, poor energy density, large footprint requirements and/or insufficient availability. Alternatively, microalgae, which exhibit high growth rates and oil content compared to higher plants and have and have the ability to grow in a range of diverse environments, have been used to produce biofuels. In particular, cyanobacteria use solar energy to convert carbon dioxide and water into biofuel, making possible a near carbon neutral petrochemical alternative.
Cost-effective biofuel production from cyanobacteria is directly linked to the density of cultures within a photobioreactor and its overall volume. Currently, the simplest strategy for cultivation of large volumes of microalgae is an open racetrack-style pond exposed to ambient air and sunlight. However, due to issues related to insufficient light distribution, temperature control, nutrient delivery, contamination, and water consumption, pond operations run at low cell densities. As a result, pond strategies suffer from poor areal productivity and low overall power density. Consequently, fully enclosed photobioreactors have been designed to provide precise control over the cultivation environment and maintain growth conditions. However, a central problem common to both open and closed cultivation strategies remains the efficient delivery of light to the microorganisms. As cultures increase in both volume and density, it becomes increasingly difficult for light to be distributed evenly to the individual bacteria. In current reactors, areas near the exposed surface tend to be overexposed, resulting in photoinhibition, and large interior regions are effectively in darkness. Flowing, dilute solutions must be employed to circulate bacteria through regions with productive light levels, placing a fundamental limit on culture density and overall power density of this technology.
A variety of photobioreactor strategies have been developed to provide more effective light distribution to cells by spatially diluting the light over a larger surface area. One strategy is to use light guides to channel light into the reactor volume and subsequently scatter the light into the media. Our reported approach employed cylindrical glass light distributors inserted into a culture tank to assist in distributing light. Sunlight harvested from arrays of Fresnel lenses was channeled to the reactor via optical fiber. Another reported approach used side-lit optical fibers inserted into the culture tank to improve light delivery. Another used optical fibers inserted into the culture chambers with the goal of scattering light collected from external solar collectors into the culture. Although these early studies indicate that increasing control and irradiated surface area within a photobioreactor can improve productivity, these technologies do not escape the fundamental limitation posed by the overexposure and shadowing issues accompanying direct irradiation of bulk cultures.
An evanescent field is a nearfield standing wave having an intensity that exhibits exponential decay with distance from the boundary at which the wave was formed. Evanescent waves are formed when waves traveling in a medium (e.g., an optical waveguide) via total internal reflection strike the boundary at an angle greater than the critical angle. Evanescent field phenomena are well known in the art.
In view of the problems and shortcomings identified above and known in the art, the embodied invention provides solutions and advantageous approaches that will benefit and advance the state of the art in this and related technologies.